Roof supports of mining installations often have a roof canopy composed of a main cap supported on hydraulic props, and an auxiliary cap which can be swung up and down at the front of the main cap. A sliding cap or extension may be mounted to the auxiliary cap and is extended from time to time in accordance with the advancement of a conveyor. In this way the roof canopy can be progressively advanced to follow up the winning progress and prevent roof collapse. In installations of this type there is a risk that if the auxiliary cap becomes too steeply inclined due to an undulating or broken roof the extension will be displaced downwardly into the path of the machinery working on the face. Hydraulic controls are known which function to prevent the extension from being advanced if the auxiliary cap is inclined at more than a predetermined extent. For example, DE-GM 82 24 441 and DE-OS 3 401 003 describe a shut-off valve that is operated mechanically in accordance with the angle of inclination of the auxiliary cap. Such controls are prone to certain difficulties because they require accurate mountings and excessive clearance in the main pivot joints tends to prevent this.
A general object of the present invention is to provide an improved form of control device.